Galaxy S4: Invalid imei

I just did a Factory Reset in my Samsung Galaxy S4 ( 4.2.2 )
Then I realized that the IMEI became invalid or unknown.
It says Invalid IMEI on the notification bar and Unknown in the IMEI information.
NEED HELP ASAP TY

I think the unusual nature of the problem is why there is no responses. I don't know how to revert the IMEI back, and resets should not affect it. Flashing ROMs doesn't affect it. I agree with Almeuit on taking it to your carrier, and see what they can do.